Conventional soldering systems, such as systems which have included reflow soldering apparatus, are used in the assembly of miniature electronic circuit components to circuit boards. Such an apparatus is typically used to affix the circuit components, such as surface mount devices, to the circuit boards. Such apparatus includes an electrically energizable thermod for reflowing solder that has been pre-deposited on the circuit board and/or the leads of the surface mount device.
There are many different types and kinds of thermods employed for providing sufficient heat to cause the reflowing of solder Such thermods have included small metal heating blades, which are connected electrically to a reflow power supply system for providing the required electrical current to heat the blades to soldering temperatures. The heated blades of the thermod cause the pre-deposited solder to flow, and connect the component or device to the circuit board when the solder is subsequently cooled and solidified.
One particular type of thermod includes a set of four blades configured in a generally rectangular or boxlike configuration, for connecting to a circuit board a conventional rectangularly shaped electronic component, such as an integrated circuit chip. The heating elements are typically electrically connected in parallel. However, such an electrical connection can cause a non-uniform electrical current distribution, and thus an undesirable temperature distribution. The resulting solder connection can be improper, and thus unacceptable. The costly assembled board can then be determined to be a reject.
The typical problem is compounded by the conventional thermod electrically conductive material used in the heating element construction. The material can exhibit a negative resistance versus temperature characteristic, such that as the temperature of an element increases, its resistance decreases. Therefore, in a parallel connection, if one element reaches a substantially higher temperature relative to its counterparts, the hottest element tends to draw substantially more current because of its lower resistance, which, in turn, causes it to heat even further. This runaway phenomenon can cause non-uniform heating by the thermod. Thus, for example, if a four-sided electronic component is being mounted to a typical electronic circuit board, one side of the component receives excessive heat which may either damage the component, or damage the circuit board. The other three elements receive insufficient heat, which may cause improper attachment of the component leads to the circuit board.
Therefore, it would be highly desirable to have a controlled soldering system power supply apparatus and method of using it, to provide uniform heating without causing damage to the delicate electronic components of the electronic chip, and without causing improper attachments of the leads of the chip to the circuit board.
Another problem associated with conventional reflow power supply systems has been the difficulty of using a single power source system for heating a variety of different kinds of reflow solder blades. In this regard, with the advent of new heat conductive materials, certain reflow solder thermods exhibit negative temperature coefficient characteristics, while others exhibit positive temperature coefficient characteristics. In other words, some materials become less electrically resistive, as their temperature rises, while other materials become more electrically resistive as their temperature rises.
In order to change thermods from one type of soldering application to another, it would be necessary to change power supplies. Also, different electrical parameters must be varied, depending on the type of material used in the thermod. For example, when the thermod material is composed of a negative temperature coefficient material, when the resistance of the material decreases with increasing temperature, the current to the thermod should be increased with increasing temperature, if the voltage output of the supply is to remain constant. Alternately, the voltage output of the reflow solder power supply should be decreased with increasing temperature, if the current is to remain constant.
In a similar manner, when the thermod material is composed of a positive temperature coefficient material, when the resistance of the material increases with increasing temperature, the current to the thermod should be decreased with increasing temperature to maintain the voltage output of the supply constant. Alternatively, the voltage output of the supply should be increased with increasing temperature, if the current is to remain constant. As a result of the foregoing, most reflow soldering systems must be limited to one type of thermod or the other. Thus, the soldering capabilities of the system are limited. In order to increase the capabilities of such system, a multiple number of power supply types would be required. The power supplies would either serve as a constant current source, or as a constant voltage source. However, such duplication of power supplies makes the system more expensive to manufacture and maintain, and also makes the system more prone to operator errors if the operator fails to change power supplies according to the type of soldering head being employed. The change over time required during the operation of the system, can produce undesirable and unwanted delays.
Therefore, it would be highly desirable to have a new and improved controlled power supply apparatus, which could be used with a variety of different kinds of thermods, composed of a variety of different types of materials, both negative and positive temperature co-efficient materials, for a variety of applications. Also, it is important to eliminate the need for a duplication of power supplies.